Wnt signaling through the canonical a-catenin pathway controls cell fate determination and cell proliferation, and is essential for animal development. Defective Wnt/b-catenin signaling has been associated with human colorectal cancer, familial osteoporosis, and other diseases. Thus understanding Wnt/b-catenin signaling is highly relevant to human health. Wnt/b-catenin signaling is initiated by 2 distinct families of cell surface receptors. One (1) is a member of the Frizzled (Fz) family of serpentine receptors, and the other is a single transmembrane receptor of the LDL receptor related protein family, LRP5 or LRP6. How Wnt leads to the activation of these receptors is a critical but poorly understood issue. Dr. He's group showed that Fz and LRP5/6 form a Wnt-inducible co-receptor complex in vitro, and that LRP6 plays a key role in the signaling process. They recently discovered that phosphorylation likely underlies LRP6 activation. They found that a PPP(S/T)P motif, which is reiterated 5 times in the intracellular domain of LRP5/6, is essential for LRP6 signaling function. They demonstrated that phosphorylation of this PPP(S/T)P motif is required for signaling and for binding/recognition by Axin, a key scaffolding protein that regulates b-catenin stability. They further showed that Wnt induces LRP6 phosphorylation in vivo. These results suggest that Wnt activates transmembrane signaling via inducing LRP6 phosphorylation. In this application, 3 specific aims are designed to substantiate/extend this working model. (1) To characterize fully all five PPP(S/T)P motifs in LRP6 intracellular domain. Issues to be addressed include the function of these PPP(S/T)P motifs and whether their differential phosphorylation controls LRP6 signaling. (2) To investigate Axin interaction with the phosphorylated PPP(S/T)P motifs. The aim is to identify the Axin domain/module that recognizes PPP(S/T)P phosphorylation, and examine whether LRP6-Axin association alters the composition of the Axin complex, thereby governing b-catenin stability. (3) To identify kinase (or kinases) that phosphorylates the PPP(S/T)P motif. This aim is to investigate known kinases implicated in Wnt/a-catenin signaling for their potential roles in PPP(S/T)P phosphorylation and to isolate the PPP(S/T)P kinase or kinases via several complementary approaches. These experiments will significantly enhance the understanding of Wnt receptor activation and Wnt signal transduction in development and disease.